The present invention relates generally to systems and methods for modifying the intensity distribution of a light beam, and more particularly to a system and a method for reducing the time average variation in the beam intensity caused by imperfections of an optical element or optical components of the optical system through which the light beam is directed.
Excimer lasers have been used for various applications, including tissue ablation such as corneal ablation and other surgical procedures. The cross-section of the intensity profile of a typical excimer laser beam is typically not spatially uniform. In general, the beam has a generally rectangular cross-section. The intensity along the long axis of the rectangular beam is substantially constant over the central portion of the beam. The intensity along the short axis of the beam is substantially Gaussian. The divergence of the excimer laser beam is different along the two axes. As a result, the beam changes shape as it travels away from the excimer laser.
Producing a laser beam with a substantially uniform intensity is important in many surgical procedures such as tissue ablation, particularly in corneal ablation for refractive correction or therapeutic purposes. In addition, the laser beam should maintain the shape required by the ablation algorithm throughout the ablation procedure.
Various methods have been used to modify the spatial energy intensity distribution of laser beams to generate a beam with more uniform intensity over the beam cross-section at the plane in which the ablation takes place. These methods invariably employ optical elements or systems for focusing, profiling, reflecting, diffracting, imaging, or otherwise optically manipulating the beam. Optical elements are further used to direct the light beam to the desired location such as a surgical site or ablation plane. Examples of optical elements include lenses, mirrors, diffractive optical members, and the like.
Optical elements may have imperfections formed during manufacturing (e.g., scratches, material variations, surface textures, or the like) or caused by artifacts on the surfaces or formed by the interaction of the laser beam with the optical element (compaction, color center formation, or surface degradation). Ultraviolet laser beams cause irregularities in all optics over time. Color centers develop in transparent optics, which then absorb some of the beam. Anti-reflective and high-reflective coatings change or sustain damage. Fused silica transmission optics are compacted in the beam area and change shape. Any imperfection of an optical element, however small, will produce a distortion of a light beam directed through the optical element or reflected by the optical element when the imperfection is disposed in the path of the light beam.
Some optical defects that cause patterns that can be seen in an ablation are very difficult to detect by conventional optical or surface measurements of the optical element. Often, it is very difficult to determine if an optical element has such defects other than by installing the optical element and testing it in a system. This kind of trial testing is time-consuming and can add significant cost to a commercial laser system.
Embodiments of the present invention provide methods and systems for temporally smoothing a distortion of a light beam caused by an imperfection or imperfections of an optical element or optical system in the path of the light beam. The optical element is moved relative to the light beam to change the position of an imperfection in the path of the light beam to spatially distribute over time the distortion of the light beam caused by the imperfections of the optical element. The optical path of the beam is stable so that moving the optical element does not substantially alter the path of the light beam. In addition, moving an optical element to distribute the effect of the imperfections can generally reduce significantly both the rate at which damage occurs to an optical element and the appearance of resulting irregularities in the beam at the final treatment plane.
In accordance with an aspect of the present invention, a method of temporally smoothing a distortion of a light beam includes directing a light beam through an optical system comprising an optical element disposed along a light beam path. The optical element has an imperfection in the path of the light beam causing distortion of an energy profile distribution of the light beam. Moving the optical element with respect to the light beam path changes the position of the imperfection in the path of the light beam, and distributes the distortion caused by the imperfection. The method may include stabilizing the path of the beam as the optical element is moved, and controlling the range of motion of the optical element to be about twice the width of the beam intersecting the element.
In some embodiments, the optical element is axially symmetric with respect to its optical axis, and the distortion introduced into the light beam by the imperfection is distributed by rotating the optical element around its optical axis. The path of the light beam may be parallel or oblique to the axis of symmetry of the optical element. The optical element may be selected from the group consisting of a lens, a mirror, a beam splitter, a transmitting plate, a prism and a diffractive optic.
In some embodiments, the optical element has geometric uniformity relative to a plane, and the distortion of the light beam is distributed by moving the optical element along the plane. The optical element may be moved by translation in at least one direction along the plane. The planar optical element may be moved by rotation around an axis which is perpendicular to the plane. In specific embodiments, the optical element is a planar optical element. The optical element may be selected from the group consisting of a planar mirror, a planar beam splitter, a planar transmitting element, a prism and a planar diffractive optic. In one embodiment, the light beam intersects the optical element over an intersecting surface area of the element, and the range of motion of optical element is less than about 50% of a dimension across the intersecting surface area.
In addition to planar optics such as planar mirrors, planar diffusers, and planar diffractive optics, different optical elements may be used. For instance, other embodiments of the present invention may employ optics which have linear symmetry along a line in a plane, such as cylindrical lenses and mirrors, and linear or cylindrical diffractive optics (e.g., diffractive gratings). For an optical element having geometrical uniformity relative to a line of symmetry in a plane, a driver may oscillate the optical element back and forth along the line of symmetry or move the element slowly in one direction along the line of symmetry and then return it and repeat the motion. The distortion in the light beam is distributed by moving the optical element along the line of symmetry.
In accordance with another aspect of the present invention, a system for temporally smoothing a light beam comprises a light source making a beam of light energy, and an optical element disposed in a path of the light beam. The optical element has an imperfection in the path of the light beam causing a distortion of the energy distribution of the light beam. An optical element driver is coupled with the optical element to move the optical element with respect to the light beam to change a position of the imperfection in the path of the light beam. The change in position of the imperfection distributes the distortion of the light beam caused by the imperfection of the optical element. The path of the light beam may be stable as the driver changes the position of the imperfection. The driver may control a range of motion of the of the optical element to be less than about twice a width of the beam as the beam intersects the element. The light source may be a pulsed laser.
In some embodiments, the optical element has geometric uniformity relative to a plane, and the optical element driver is configured to move the optical element along the plane. In some embodiments, the optical element is axially symmetric with respect to its optical axis, and the optical element driver is configured to rotate the optical element around the optical axis of symmetry thereof. In an embodiment, the light beam intersects the optical element over an intersecting surface area of the element, and the range of motion of the element is less than about 50% of a dimension across the intersecting surface area.
Another aspect of the invention is directed to a method of smoothing an ablation in a material at a treatment plane using a pulsed laser beam. The method comprises making a pulsed laser beam and directing the beam through an optical system comprising an optical element disposed along a laser beam path that directs the laser beam to the treatment plane. The optical element has an imperfection in the path of the laser beam causing distortion of the laser beam. Moving the optical element with respect to the laser beam to changes the position of the imperfection in the path of the laser beam and distributes the distortion caused by the imperfection. The beam ablates the material to form the ablation. The method may include stabilizing the path of the laser beam while moving the optical element. The method may also include controlling a range of motion of the moving element to be less than about twice a width of the beam. The ablation material may be corneal material and the laser may be an excimer laser.
Another aspect of the present invention is directed to a system for forming an ablation at a treatment plane using a pulsed laser beam. The system comprises a pulsed laser source for making a pulsed laser beam. An optical system comprises an optical element disposed along a laser beam path that directs the laser beam to the treatment plane. The optical element has an imperfection in the path of the laser beam causing distortion of the laser beam. An optical element driver coupled with the optical element changes a position of the imperfection in the path of the laser beam and distributes the distortion caused by the imperfection.